Side by side with block copolymers, graft copolymers having a comb-like structure have attracted considerable attention in the field of macromolecular materials for the reason that these polymers have characteristics of constituent segments, as can be seen with thermoplastic elastomers and shock resistant plastics, and at the same time can express unique functions based on their microscopic phase separation structure.
Graft polymers have long been used in modifying polymers. It is, however, only recently that polymers with a well controlled structure were successfully synthesized. The concept of “macromolecular monomers” was developed by Milkovich and his colleagues and, by copolymerizing such monomers, it is now possible to synthesize polymers having a well-defined comb-like structure.
On the other hand, stellar polymers have linear polymer arms radially extending from the core thereof are known to have various properties distinct from those of linear polymers.
Roughly classified, two methods are available for the synthesis of stellar polymers. One method comprises causing arm polymers to grow from a compound or polymer, which serves as a core, while the other comprises first preparing polymer arms and then joining them together to form a stellar structure. For joining arms together, there may be mentioned the technique comprising reacting a compound having a plurality of functional groups capable of reacting with the terminal functional groups of the arm polymers, the technique comprising adding a compound having a plurality of polymerizable groups after preparation of arms by polymerization, and the technique comprising polymerizing a polymer having a terminal polymerizable group (hereinafter referred to as “macromonomer”), among others.
Such stellar polymers are constituted of homopolymers and copolymers of various kinds, such as polystyrenes, poly(meth)acrylates, polydienes, polyethers, polyesters and polysiloxanes. For obtaining a controlled stellar structure, it is necessary, irrespective of method of production, for the polymerization to be controlled. Therefore, the anionic polymerization, living cationic polymerization or polycondensation technique is employed in most instances.
Contrary to those polymers obtainable by ionic polymerization or polycondensation, such as specifically mentioned above, those vinyl polymers which are obtainable by radical polymerization and have a stellar structure have scarcely been put to practical use. In particular, any method has not yet been successfully developed for causing chain extension or constructing a stellar structure through binding of macromonomer molecules. Generally, vinyl polymers have those characteristics which the above-mentioned polyether polymers, hydrocarbon polymers or polyester polymers cannot have, for example high weathering resistance and transparency and, therefore, those having an alkenyl group or a crosslinkable silyl group on a side chain thereof are utilized in high weathering resistance coating compositions, for instance.
While graft polymers and stellar polymers can be obtained by using macromonomers, it is not yet easy to synthesize the macromonomers. In particular, it is difficult to control the polymerization in preparing vinyl polymer macromonomers to be generally subjected to radical polymerization, hence few such macromonomers have been synthesized. It is not easy, because of side reactions, to control the polymerization of acrylic polymers, among others, and, therefore, it is difficult to produce macromonomers having a terminal polymerizable group.
Accordingly, it is an object of the invention to provide a production method of a branched polymer using vinyl polymer macromonomers prepared by radical polymerization.